Default values don't work for grouped parameters

I added a spec that fails because default values are not present for grouped parameters. I'll try to implement a fix and add it to this PR... 
It seems like the default-value-validator isn't aware that it's handling a grouped parameter. Most probably it doesn't have to, but I suppose this might somehow cause the default value to be ignored.

If anyone knows an easy way how to fix this or sees the problem, any help is appreciated!